These teachings relate to electro-mechanical measurement and control systems.
One class of position sensors commonly used in feedback control systems are linear variable differential transformer (LVDT) sensors. These sensors consists of a cylindrical array of a primary and typically two secondary windings where a cylindrical core passes through the centre of each of the windings. The primary windings is energized with a constant amplitude A.C. supply at a frequency of 1 to 10 kHz. This produces an alternating magnetic field in the centre of the transducer which induces a signal into the secondary windings that is a function of the position of the cylindrical core relative to the primary and secondary windings. Typically, when the core is positioned at the centre of the two secondary windings, a zero signal is derived. Movement of the core from this point in either direction causes the signal to increase. As the windings are wound in a particular precise manner, the signal output has a linear relationship with the actual mechanical movement of the core. The secondary output signal is then processed by a phase-sensitive demodulator which is switched at the same frequency as the primary energizing supply. This results in a final output which, after rectification and filtering, gives an output proportional to the core movement and also indicates its direction, positive or negative from the central zero point.
However, the LVDT output may not be sufficiently robust in terms of dynamic range, and moreover the output may also be adversely affected by cable capacitance and thermal drift. What is needed in the art then is a position sensor that is a low cost replacement for existing LVDT and is more resilient in terms of the sensor output being affected by temperature and cable capacitance.